1. Field of the Invention
The present invention relates to a piezoelectric rotating motor which generates the mechanical power by the utilization of ultrasonic vibrations generated by the piezoelective effect of piezoelectric elements.
2. Description of the Prior Art
Piezoelectric rotating motors arc made up of a stationary part, or stator, and of a rotating part, or rotor. The stator and the rotor contact with each other under pressure along circular friction zones which are positioned at their peripheral portions. Piezoelectric elements are arranged in the vicinity of the friction zone of the stator. For example, the piezoelectric elements of the piezoelectric rotating motor disclosed in U.S. Pat. No. 4,562,374 are fixed to the lower face of the stator which is formed the circular friction zone at its upper face, by adhesives and so on. High-frequency voltages with proper different phase are applied to each of these piezoelectric elements, respectively and thereby a progressive wave forming an elliptical vibration is produced on the friction zone by ultrasonic vibrations of the piezoelectric elements. This progressive wave is amplified by comb-shaped teeth which are formed on the friction zone and drives the rotor by the frictional force. Thereby, the mechanical power is obtained as a rotational force of the rotor. The piezoelectric rotating motors have the advantage, in particular, of providing a high torque with a low rotational speed.
In the above prior motors, however, the power of motors is limited by the fact that the piezoelectric elements are likely to break if subjected to excessive bending. Furthermore, the motors undergo many energy losses due to rubbing, leading in addition to overheating that are harmful to the good working of the piezoelectric elements.
A piezoelectric rotating motor which overcomes the above drawbacks is proposed by Japanese patent application laid-open publication No. 1(1989)-117671. In this motor, ring-shaped piezoelectric elements are sandwiched between a base plate and a disk-shaped stator having a diameter which is larger than that of the piezoelectric elements. A friction zone is formed on an outer peripheral portion of the stator which is outwardly apart from the piezoelectric elements in the diametrical direction. According to this motor, it is able to overcome the above drawbacks and furthermore it is possible to produce a large amplitude on the friction zone, because the amplitude of the piezoelectric elements is amplified by the leverage.
In the above prior piezoelectric rotating motor, however, since the thickness of the stator is uniformity, the vibration energy of the piezoelectric elements is apt to transmit inward in the diametrical direction and it is not able to effectively transmit the vibration energy of the piezoelectric elements to the friction zone. This is remarkable, when the rotor is contacted with the friction zone under pressure.